Synthetic resins such as phenolic resins have enjoyed widespread use for many years. Such resins are obtained, for instance, from the condensation of phenol, or substituted phenols, with aldehydes such as formaldehyde, acid aldehyde, or furfural. Phenol-formaldehyde resins are typical of the group, constituting the chief class of phenolics, and when cured, the same are resistant to moisture, solvents, heat, and the like. Such resins are also dimensionally stable, have good electrical resistance, in addition to being non-combustible, and they possess good sound and noise absorbing characteristics. In view of these and other desirable properties, the resins find widespread use for fabricating molded and cast articles, in bonding powders, for ion exchange purposes, in laminating and impregnating operations, in manufacturing composites and electrical components, as well as many other areas.
While phenolic resins have been employed satisfactorily in the areas mentioned, they are not without certain drawbacks. For example, since their synthesis involves a condensation-type polymerization, they produce condensation by-products, which in many applications are a distinct disadvantage. Furthermore, in the case of the preparation of Novolak resins, for example, the synthesis depends upon the use of undesirable strong acid catalysts.
As a consequence of the foregoing and other disadvantages, in some instances, resort has been had to resins prepared from monomeric benzoxazine compounds, since they not only eliminate the need to use strong acid catalysts, but inasmuch as their polymerization is accomplished through the use of a ring-opening polymerization, they do not involve the generation of objectionable condensation byproducts. Furthermore, they are characterized by possessing a relatively long shelf life, and in addition, they offer considerable molecular-design flexibility, and have comparatively low viscosities, the latter property being one which is beneficial in processing them into desired products. In view of the many advantages derived from the use of benzoxazine compounds, their employment in the preparation of resins has become increasingly attractive.
Until now, however, benzoxazine resins have commonly been synthesized by dissolving the components necessary for their preparation, that is the phenolic derivative, an aldehyde, and a primary amine in a suitable solvent such as dioxan, toluene, alcohol, and the like. Such a synthesis is shown, for example, in British Patent No. 694,489 in which 3,4-dihydro-1,3 benzoxazine is said to be preparable in a solvent such as dioxan or in a lower aliphatic, water miscible alcohol, for example, methanol or ethanol, from paraformaldehyde in combination with certain substituted phenols and aliphatic primary amines. The desired product is subsequently isolated from the solvent medium by evaporation of the solvent, or by precipitating the synthesized product from solution through the addition to the reaction mixture of a suitable non-solvent.
Similarly, U.S. Pat. Nos. 5,152,939 and 5,266,695 are concerned with carbon-carbon composites prepared from benzoxazine compounds formed in solutions in which the reactive components are dissolved.
While the preparation of benzoxazine compounds from reaction mixtures in which the reactive components are present in solution has previously been used, such methods are not without difficulties. For example, it usually takes a relatively long time, i.e., at least several hours, to carry out the desired reaction, and to separate the reaction products from the solvent present. Furthermore, additional time is required if purification is necessary, as is often the case. In addition, although the yield of benzoxazines is satisfactory in reactions where solvents are present, the solvents pose toxicity risks, which in many cases require expensive measures to eliminate, for instance the installation of costly solvent recovery systems. Still further, as a consequence of the cost of replacing fugitive solvents, and for other reasons, the elimination of solvents from the reaction mixture altogether would provide an important economy to the synthesis process.
In recognition of the health risks and cost disadvantages described, therefore, some effort has in the past been devoted to the preparation of solventless systems. Kopf and Wagner, for example, have prepared the benzoxazine of 2,4-xylenol by heating hexamethylenetetramine and 2,4 xylenol in air for 21/2 hours. However, this method is relatively expensive since it involves two stages, the first of which requires the synthesis of hexamethylene tetraamine from ammonium and formaldehyde, and subsequently, a further reaction between hexamethylene tetraamine and 2,4-xylenol; P. W. Kopf and E. R. Wagner, J. Polym. Sci., Polym. Chem. Edi. 11, 939 (1973). However, while solventless syntheses with two reactants are relatively common, a one-step synthesis for a product involving three reactants is unusual.
In view of the preceding, therefore, it is a first aspect of this invention to provide a solventless synthesis for the preparation of benzoxazine compounds in a solventless system.
A second aspect of this invention is to provide a single step method for the preparation of benzoxazine compounds.
Another aspect of this invention is to provide a solventless benzoxazine synthesis that is faster than a benzoxazine synthesis employing a solvent in the reaction mixture.
A further aspect of this invention is to provide a solventless benzoxazine synthesis in which the yield of benzoxazine is substantially as good as a benzoxazine synthesis that requires the use of a solvent medium to dissolve the reaction components.
An additional aspect of the invention is to provide a benzoxazine synthesis in which toxic solvents are eliminated.
Yet another aspect of this invention is to provide a benzoxazine synthesis which has fewer byproducts than a comparable synthesis in which, however, a solvent is employed in the reaction mixture.
A still further aspect of this invention is to provide a reaction mixture for synthesizing benzoxazine in which the concentration of reactants in the reaction mixture is maximized.
Still another aspect of this invention is to provide a continuous method for synthesizing benzoxazines in the absence of a solvent medium.